This invention relates to determining the position and orientation of a remote object with respect to a reference point; and, more particularly, to radiating an electromagnetic field from the reference point, detecting the field at the remote object and analyzing the detected field to determine the position and orientation of the remote object.
The use of orthogonal coils for generating and sensing magnetic fields is well known. For example, such apparatus has received wide attention in the area of mapping magnetic fields to provide a better understanding of their characteristics. If a magnetic field around generating coils can be very accurately mapped through use of sensing coils, it has also been perceived that it might be possible to determine the location of the sensing coils relative to the generating coils based on what is sensed. However, a problem associated with doing this is that there is more than one location and/or orientation within a usual magnetic dipole field that will provide the same characteristic sensing signals in a sensing coil. In order to use a magnetic field for this purpose, additional information must therefore be provided.
One approach to provide the additional information required for this purpose is to have the generating and sensing coils move with respect to each other, such as is taught in U.S. Pat. No. 3,644,825. The motion of the coils generates changes in the magnetic field, and the resulting signals then may be used to determine direction of the movement or the relative position of the generating and sensing coils. While such an approach removes some ambiguity about the position on the basis of the field sensed, its accuracy is dependent on the relative motion, and it cannot be used at all without the relative motion.
Another approach that has been suggested to provide the additional required information is to make the magnetic field rotate as taught in Kalmus, "a New Guiding and Tracking System," IRE Transactions on Aerospace and Navigational Electronics, March 1962, pages 7-10. To determine the distance between a generating and a sensing coil accurately, that approach requires that the relative orientation of the coils be maintained constant. It therefore cannot be used to determine both the relative translation and relative orientation of the generating and sensing coils.
U.S. Pat. No. 3,868,565, assigned to the same assignee, teaches a tracking system for continuously determining at the origin of a reference coordinate system the relative translation and orientation of a remote object. The tracking system includes radiating and sensing antenna arrays each having three orthogonally positioned loops. Properly controlled excitation of the radiating antenna array allows the instantaneous composite radiated electromagnetic field to be equivalent to that of a single loop antenna oriented in any desired direction. Further control of the excitation causes the radiated field to nutate about an axis denoted a pointing vector. This tracking system is operated as a closed-loop system with a computer controlling the radiated-field orientation and interpreting the measurements made at the sensing antenna array. That is, an information feedback loop from the sensing antenna array to the radiating antenna array provides information for pointing the nutation axis toward the sensing antenna array. Accordingly, the pointing vector gives the direction to the sensing antenna array from the radiating antenna array. The proper orientation of the pointing vector is necessary for computation of the orientation of the remote object. The signals detected at the sensing antenna include a nutation component. The nutating field produces a different nutation component in each of the three detected signals. The orientation of the sensing antenna array relative to the radiated signals is determined from the magnitudes of these components.
U.S. Pat. No. 4,054,881, assigned to the same assignee, teaches a magnetic or near-field non-tracking system for determining, at a remote object, the position of the remote object with respect to a reference coordinate system. The orientation of the remote object can be determined, at the remote object, with respect to the reference coordinate system by using an iterative computational scheme. This is accomplished by applying electrical signals to each of three mutually orthogonal radiation antennas, the electrical signals being multiplexed with respect to each other and containing information characterizing the polarity and magnetic moment of the radiated electromagnetic fields. The radiated fields are detected and measured by three mutually orthogonal receiving antennas, having a known relationship to the remote object, which produces nine parameters. These nine parameters, in combination with one known position or orientation parameter are sufficient to determine the position and orientation parameters of the receiving antennas with respect to the position and orientation of the radiating antennas.
Copending and allowed U.S. Patent application, Ser. No. 62,140 filed July 30, 1979 entitled REMOTE OBJECT POSITION AND ORIENTATION LOCATER, and assigned to the same assignee, teaches several improvements to U.S. Pat. No. 4,054,881. Relating to the definition of a minimum excitation set in the near field context. In particular two-state excitation and two axis transmission or reception is taught in a non-tracking system that determines orientation in a non-iterative manner. However, as in all the aforementioned disclosures, the distance between the radiating antennas and the receiving antennas is again limited to the region in which the near field is dominant.
Copening U.S. Pat. No. 4,298,874, issued Nov. 3, 1981, assigned to the same assignee and entitled METHOD AND APPARATUS FOR TRACKING OBJECTS, teaches a tracking system for: (a) determining at the origin of a first body coordinate reference frame the relative position and orientation of a second body and, (b) determining at the origin of a second body coordinate reference frame the relative position and orientation of the first body. The separation distance between the bodies is not limited to the near field. Each body of the tracking system includes at least two independently oriented stub dipoles for radiating and sensing electromagnetic fields. Properly controlled excitation of the radiating antenna allows the radiated field to nutate about an axis denoted a pointing vector. The first body receives radiation transmitted from the second body and establishes the pointing angles to the second body with respect to the first body coordinate reference frame. The processing which determines the pointing angles is dependent on the fact that no modulation or nutation components exist in the radial direction. The field received by the first body can include information defining the second body's pointing angles to the first body with respect to the second body's coordinate reference frame and the relative roll about their mutually aligned pointing axes. This information is sufficient for determining the orientation of the first body relative to the second. This process is then repeated with the second body receiving radiation transmitted from the first body. Further, information can be transmitted from the first body to the second body which establishes a vector from the second body to a third body, thus defining the location of the third body at the second body.
Copending U.S. patent application Ser. No. 137,287, filed Apr. 4, 1980, assigned to the same assignee and entitled LOCATOR FOR SOURCE OF ELECTROMAGNETIC RADIATION HAVING UNKNOWN STRUCTURE OR ORIENTATION improves upon the aforementioned system by teaching means for determining the position of the source relative to the receiving means without a priori knowledge of the orientation of the source or the relative orientation of its components.
While the art of determining the position and orientation of remote objects is a well developed one, operation of near-field systems over large ranges requires impractically large antennas and impractically high transmitter powers. And, there still remains a need to determine the position and orientation of a remote object in the context of a non-tracking system, employing a minimum excitation set, when the separation distance from the remote object to a reference coordinate frame is such that the far-field component of the transmitted radiation substantially exceeds the near-field component of the radiation. There is also a need for determining, in the context of a far-field non-tracking system, the position and orientation of a remote object with respect to a reference coordinate frame by the use of either two radiating antennas and three receiving antennas or three radiating antennas and two receiving antennas.